CLASSES OF COMPRESSION MEMBERS 44. Compression members are those which are stressed by loads or forces at their ends, and these forces act towards each other. If a man pushes on his cane, which has the other end resting upon something, the cane is said to be in compression, and it is a compression mem ber. This is the direct opposite of the tension member, which is illustrated by a string which has a pull exerted at each end.
Compression members may be made of dif ferent shapes or combinations of shapes. They may consist of single shapes such as a single angle, I-beam, channel, Z-bar, etc., or of two or more angles, two or more Z-bars, or two chan nels or two I-beams, each combination with or without a plate connecting them. In case plates are not used, the shapes are connected with tie plates, or with lacing bars and tie-plates. Fig, 65 shows some of the different classes of com pression members.
45. Tie-Plates and Lacing Bars. The size of these is not determined by design, but rather by common practice. The size of the tie-plates, often called batten plates, is usually limited by the specification, but it is good practice never to have the distance x (Fig• 65, e) less than w when the plates are at the end of the member. When there is no lacing in use, the shapes being held together by plates as in Fig. 65, f, the distance x may be one-half of w. With the better class of designers, it is now becoming customary to make x (Fig. 65, e) times w.
The sizes of lacing bars which have been proved to be efficient, depend upon the angles they make with the axis of the member, and upon their length. Table XV gives the sizes of such lacing bars: Some specifications give the widths to be used. In case they do not, the above may be taken.